1. Field of the Invention
This invention relates to a semiconductor laser driving device and optical disc device, and in particular relates to a semiconductor laser device capable of adjusting the laser optical power from the semiconductor laser, and an optical disc device using this semiconductor laser device.
2. Description of the Related Art
Semiconductor laser LDs for recording and reproduction are mounted in conventional CD drives, DVD drives, and other optical disc devices, and the power and pulse width of the laser light emitted from the semiconductor laser are adjusted. With respect to laser light power in particular, a photodiode PD is positioned near the LD to receive laser light from the LD and detect the power, and an APC (auto-power control circuit) is used to drive the LD so as to coincide with a target value.
On the other hand, in DVD-R drives, DVD-RW drives and similar, data is recorded using multi-pulse recording. That is, there exist 3T to 11T and 14T recording data marks (where T is the basic data length), and the shortest data mark 3T is recorded using a single pulse, but 4T and longer data marks are recorded using multi-pulse recording by a leading pulse and a following pulse succeeding this.
An example of a multi-pulse signal is shown in FIG. 6. Read power (reproduction power) is superposed on level 0, and write power (recording power) is further superposed on the read power; the write power is formed from the leading pulse and a following pulse. By irradiating the DVD-R disc or similar with such multi-pulse laser light, pits are formed on the DVD-R recording film (for example, an organic dye film). In the figure, pit widths are determined according to the pulse widths of the multi-pulse signal, but in actuality, because there is thermal diffusion in the recording film, the pit widths are somewhat larger or smaller. Also, in the figure the leading pulse has a constant write power, but in order to make the recording edge sharper, the rising portion of the leading pulse is boosted above the write power.
Conventionally, when performing data recording using multi-pulse signals, the average value of a signal from the PD and the target value thereof are compared in feedback control of the LD power; a technique has been proposed to correct one of the average value and the target value according to the change in the pulse duty ratio (see for example Japanese Laid-open Patent Application No. 2002-57403). In the description of this patent document, when for a pit length to be recorded the pulse duty changes from 50% to 40%, by subtracting a predetermined amount from the average value output from a low-pass filter or adding a predetermined amount to the target value, the write power is maintained at the target value. This correction amount is either computed by a CPU, or can be determined from a table.